Controlling or reducing the concentrations of gas-phase impurities, contaminants, or other undesirable materials through the use of chemical getters is widely practiced in a variety of industries, including silicon and compound semiconductor growth, photolithographic processing and optical fiber manufacturing, resulting in improved performance and yield. Analytical instrumentation applications include gas chromatography and moisture/oxygen analyzers. Another industry in which it can be important to remove impurities is in packaging of hermetically sealed electronic devices in which moisture, other gases, and/or particulates may cause issues with reliability, damage, and reduction of device lifetime. In general, a getter composition is placed in a volume containing one or more devices or objects that are susceptible to damage caused by contact with or exposure to one or more gas-phase or other airborne contaminants. The getter is typically a reactive solid material that either adsorbs, absorbs, chemisorbs, or catalyzes a reaction that immobilizes or destroys one or more targeted contaminant compounds.
Some examples of atmospheres and their potential gas phase contaminants include purge gases, compressed dried air (CDA), N2, O2, and mixtures with inert gases. These can be purified of contaminants such as SO2, SOx, NOx, H2S, H2O, CO2, hydrocarbons, siloxanes, ammonia, amines, and acid gases using metal oxide catalysts. Gas phase contaminants including H2O, H2, and CO2 that occur in chlorinated, brominated, and fluorinated gases may be removed using impregnated or ion exchanged zeolite adsorbent materials. Hydrogen gases, inert gases and mixtures of the same that are contaminated with H2O, O2, CO, CO2, and non-methane hydrocarbons can be purified using nickel silica and titanium catalysts, typically removing the contaminants through a combination of chemisorption, oxidative addition, and simple oxidation and adsorption.
One common application in which getters are frequently employed is optoelectronic and microelectronic devices. The use of dried air to enclose electronic packages has been standard practice for many years. In general, the air is specially dried so that the dew point of the internal atmosphere is well below the lowest storage or operational temperature of the package. However, when the package material is principally a plated metal, such as Kovar, it has been observed that over time, hydrogen evolved from the plating or the metal reacts with oxygen present in the internal atmosphere to produce water vapor. Another possible reaction path is through oxides present on the surfaces of the package or the device. In this case, the oxide is reduced and water vapor is released into the package. Given sufficient time, the amount of water vapor generated through either or both mechanisms can cause the internal atmosphere dew point to exceed the operational or storage temperature, whereupon the water vapor condenses to liquid form. Reduction of solid oxides may lead to an impairment of the device performance by altering the electrical or optical characteristics. Condensation of water can lead to contamination, corrosion, and other types of chemical and/or physical damage. Hydrogen may be present in the enclosure atmosphere as an impurity or contaminant in the gas used to fill the enclosure or it may outgas from the walls of a metal enclosure over time or as the temperature increases. Many microelectronic and optoelectronic devices are sensitive to moisture with consequent effects on reliability due to corrosion or other alteration of contact materials. Heating and cooling may compound the problem. Metal migration can occur when there is a potential difference between two adjacent conductors, in the presence of moisture and ionic contaminants. Biased ions are carried via the moisture to migrate the conductor from a lower to a higher potential conductor, thereby forming a short-circuit between the conductors. Formations such as dendritic growth and metal whiskers can occur, with detrimental effects on device functioning. Corrosion occurs when a contaminant behaves as a catalyst with water to oxidize metallization and continues until all of the available water is consumed, generally resulting in device failure.
In addition to the aforementioned problems that may be caused by water formation from reactions of oxygen with hydrogen, enclosures that include ferrous alloys and other metallic materials such as, for example, gallium arsenide are particularly sensitive to damage due to hydride formation. Metal hydrides are gradually oxidized, which may result in device failures in both silicon and gallium arsenide devices. For example, hydrogen gas concentrations as low as 0.5% of ambient atmosphere have been shown to cause significant degradation in a relatively short time (168 hours) at elevated temperatures of 125° C. in GaAs field effect transistors (FETs) and microwave monolithic integrated circuits (MMICs). Investigators have proposed that the mechanism for device damage includes catalytic conversion of the molecular hydrogen by platinum at the gate, resulting in atomic hydrogen diffusing into the semiconductor, compensating the silicon dopant donors and reducing the current and gain of the device. Considerable attention has been given to the issues surrounding the degrading effects of hydrogen on a range of GaAs microelectronic device technologies. The potential for device damage is also of concern for optoelectronic technologies. Many optoelectronic devices are fabricated using similar techniques to those in GaAs microelectronics. Thus the deleterious effects of hydrogen can be two-fold. Hydrogen “poisoning” can change the structure of the semiconductor itself, and the evolved hydrogen may react with oxygen to form water resulting in an overall reduction in the oxygen levels within the enclosure and the consequent effects of moisture as described previously. Coatings or films on optical surfaces may be negatively affected or degraded as well.
A device that is packaged in a controlled atmosphere with minimal levels of either moisture or hydrogen will not necessarily be permanently immune to the aforementioned effects. A significant contribution may be made over time by outgassing from the packaging materials. In addition, a perfect hermetic seal is not practically achievable. An enclosure with a measured helium “leak-rate” of no more than 10−8 mbar L s−1 is typically defined as being hermetically sealed. The resulting concentrations of contaminant gases in a sealed enclosure vary and, in general, increase over time. Residual gas analysis (RGA) studies on sealed empty ferrous alloy packages have demonstrated evolution of significant amounts of moisture and hydrogen over a period of 1000 hours at a temperature of 125° C. Initially, it was theorized that the source of the hydrogen was the plating processes that may have entrapped hydrogen molecules within the plated metals that subsequently diffused from the plating with thermal stress. However, later research showed that there is an additional mechanism associated with the base alloys. Test results showed that these alloys typically contain small (on the order of approximately 0.5 ppmw or parts per million by weight) amounts of hydrogen. However, when these hydrogen contents are recalculated to an equivalent gas phase concentration in the volume of a sealed enclosure, the result is an extremely high level of hydrogen (on the order of approximately 17,000 ppmv or parts per million by volume) that may be desorbed into the enclosure. The importance of hydrogen desorption increases as the volume of a sealed enclosure decreases because the surface area to volume ratio of an enclosed volume increases as its volume decreases. Desorption occurs continuously because of diffusion paths in the base alloy and cannot necessarily be blocked.
One way of mitigating the effects of hydrogen in a ferrous alloy cavity package is to anneal the package prior to sealing at a higher temperature than that at which it will be operated, such that hydrogen is desorbed over a period of time before the device is installed and the package sealed. This type of bake-out procedure can effectively reduce the quantity of hydrogen that is available for desorption at a lower temperature during the operating lifetime of the device. When used in combination with a method of gettering any remaining hydrogen within the sealed package during that lifetime, the gas-phase hydrogen concentration can be effectively reduced to approximately zero.
A number of methods and systems have been described in the prior art to reduce gaseous hydrogen desorbed within the package. Some employ metal alloys that react with hydrogen to form metallic hydrides. Many of these getters require high operating temperatures to promote quantitative conversion of hydrogen. One approach described in U.S. Pat. No. 5,888,925 to Smith et al. utilizes palladium oxide combined with a Zeolite desiccant held together using a binder such as RTV silicone to provide a catalyst that reacts hydrogen to water which is subsequently adsorbed and contained to remove free hydrogen from the sealed atmosphere. Although this system is capable of operating at low temperature, it has several potential weaknesses. The zeolite desiccant has a substantially reduced sorption capacity for water at higher temperatures—up to 70% of the capacity is effectively lost at temperatures greater than 100° C. Thus, trapped water may be liberated within the enclosure at higher operating temperatures. This substantially diminishes the protective effect. Problems associated with this physical mechanism may be reduced by increasing the overall quantity of adsorbent material. However this temperature-reversible effect is undesirable in general. Additionally, the bulk of the added getter material required to protect against release of trapped water at elevated temperatures necessitates use of greater volume within the sealed enclosure to provide the needed protection capacity. Two other prior art getters disclosed in U.S. Pat. No. 5,696,785 to Bartholomew et al and U.S. Pat. No. 6,200,494 to Manini et al use a combination of adsorbent materials, bound with a plasticizer. U.S. Pat. No. 4,081,397 to Booe discloses a similar getter device in which irreversible alkali oxide desiccants are bound into an elastomeric material. All of these prior art getters suffer from the introduction of organic materials, such as would be used for the binder, into the sealed cavity. Organic compounds are a particular concern in the packaging of optoelectronic devices, due to contamination of the optical surfaces and consequential impairment to operation.